for National Geographic News
Are we alone? The question has tickled the human imagination for years, perhaps ever since our earliest ancestors looked up at the night sky into the endless sea of stars. Is anyone else out there?
Humans do not yet have a definitive answer to the question, but a team of astronomers running computer simulations of planet formation has found that Earthlike planets with enough water to support life could be fairly common.
The team ran 44 simulations of planet formation near a sun and found that each simulation produced one to four terrestrial (rocky, Earthlike) planets, including 11 planets at about the same distance from their stars as Earth is to the sun.
"It's hard to say we sampled exactly the conditions in the galaxy in which terrestrial planets form, but in the cases we chose, an Earth-like planet formed in about a quarter of the cases," said Sean Raymond, an astronomer at the University of Washington in Seattle.
Raymond is the lead author of a paper on the simulations recently accepted for publication in the astronomy journal Icarus. Co-authors are Thomas Quinn at the University of Washington and Jonathan Lunine at the University of Arizona in Tucson.
The simulations show a wide variety of planets forming; everything from dry planets like Mars, to Earthlike planets, to planets three times as large as Earth with 20 times as much water.
Greg Laughlin, an astronomer at the University of California, Santa Cruz, and expert on planet formation, said he agrees with the interpretation that the universe may be full of a variety of planets, including Earthlike planets with significant water to harbor life.
"There is nothing in our current understanding of planet formation and planetary systems to suggest that the basic architecture of our solar system is particularly unusual," he said.
Planet Formation
Raymond and his colleagues based the computer simulations on what is known about the late stages of the planet-formation process, starting with a gas-giant body like Jupiter and a disc of smaller bodies in orbit around a star.
They then ran simulations on standard desktop personal computers, each simulation taking from a few weeks to several months to process. Inside the computer, ones and zeroes painted a picture of planetary bodies colliding, bonding, and splitting until, eventually, a group of surviving planets emerged.
"If you change your initial conditions a little bit, your final outcome will be quite different," said Raymond. This illustrates that planet formation is a chaotic process, he added.
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